Digital Realty Trust has in its 2009 10-K statement on page 35 a list of its top 20 tenants.

I've been staring at this table for a while, seeing what could be learned from this public disclosure.

Top 3 tenants are resellers of space - Savvis, Equinix and Qwest comprise 24.8% of the sq ft, but are only 18.9% of the rent.

The 3 financials - JPMorgan Chase, Morgan Stanley, and HSBC are 2.8% of the sq ft and 7.1% of the revenue which can be explained by the high availability requirements for their space and requirements to support financial transactions.

Google is not in the Top 20 listed. But, Microsoft, Yahoo! and Facebook are. Microsoft has the most amount of space of these 3 with 2.5% of the space and 1.6% of revenue. Yahoo! has 0.9% of the sq ft and 1.9% of the revenue. With Facebook at #4 in revenue with 3.6%, they have only 1.1% of the space in Digital's data centers. Facebook being a young company could be in high density space, so judging just by sq ft and not knowing power leaves more questions.

Comverse Technology looks like they have the most amount of space in one location 2.9% of the sq ft and only 1.4% of the revenue. But, all spaces are not created equal and it would seem the space Comverse got into had little demand from others which may explain why there is so much space.

Why are these numbers important? As companies decide to build data centers, they use their colocation costs as part of the business justification. But, how many look at the costs others are paying for colocation space to judge the ROI of a data center?